1. Field of the Invention
The present invention relates to lead containing and lead free solders and methods of forming solder joints using such solders. While it related to solders generally, more particularly, it also relates to solders comprising tin and copper, and solder comprising tin, sliver and copper.
2. Background Art
There has been an extensive search for Pb-free, solder, alloys in recent years. Several promising candidates have been identified for different soldering applications, which include Sn-0.7Cu, Sn-3.5Ag, Sn-3.0Ag-0.5Cu, and Sn-3.5Ag-4.8Bi (in wt.%). It is noteworthy that the compositions of most of the candidate Pb-free solders are Sn-rich solders, typically, 90 wt.% Sn or higher. This suggests that the physical, chemical, and mechanical properties of the proposed Pb-free solders are heavily influenced by the properties of pure Sn, as opposed to eutectic Sn—Pb which consists of mixtures of Sn-rich and Pb-rich phases.
The melting point of most Pb-free, commercial, solders is within the range between 208 and 227° C., which is about 30° C. to 40° C. higher than the melting point Of the Sn—Pb eutectic solder alloy. Reflow temperatures for these alloys are correspondingly higher and this fact has serious implications on the performance of packaging materials and assembly processes and can affect the integrity and/or reliability of Pb-free microelectronic packages.
Among the several Pb-free candidate solders, the near-ternary eutectic Sn—Ag—Cu (SAC) alloys, with a melting temperature of approximately 217° C., are becoming consensus candidates, especially for surface mounted card assembly, including BGA solder joints. Accordingly, extensive research and development activities are currently focused on the Sn—Ag—Cu system to understand the fundamental application issues and to evaluate the reliability risk factors associated with solder joints formed from this alloy family. In addition to the search for Pb-free solder alloys, a proper choice of a solderable layer on a substrate, lead-frame, module, or integrated circuit chip (such as under ball metallurgy (UBM) in a flip chip) is another critical factor affecting the long-term reliability of solder joints through their interfacial reactions. For Pb-free solder joints, the interfacial reactions are known to be more severe than with the eutectic, Sn—Pb, alloy, because of their high Sn content and high reflow temperature. In general, the kinetics of Cu or Ni dissolution in Sn-rich, Pb-free, solder joints is much faster and greater than with eutectic Sn—Pb solder.
Numerous investigations with Pb-free solders have been recently conducted to understand the mechanisms of their interfacial reactions. Several important factors affecting their interfacial reactions were identified, such as solder composition, minor alloying elements, solder volume-to-pad area ratio (the reaction rate will be high when the ratio is high), diffusion barrier layer, solder application method, and reflow condition.
In a previous study, a minor addition of Zn to SAC alloys was found to be effective in controlling the supercooling of the solder alloy below the melting point and, consequently, controlling the formation of large Ag3Sn plates, as well as in modifying the bulk microstructure and mechanical properties of the alloys.